1. Field of the Invention
The present invention generally relates to a focus detecting device for multi-point focus detection in which a combination of an axial focus detection area and an off-axial focus detection area is employed.
In the multi-point focus detection in which not only is the axial focus detection area employed, but the off-axial focus detection area is also employed, the use of an optical system for axial focus detection as an optical system for off-axial focus detection will bring about the following problems. Specifically, if the off-axial focus detection area is spaced a distance from the optical axis in order to avoid any possible interference between an axial light flux for the axial focus detection and an off-axial light flux for the off-axial focus detection, the off-axial focus detection optical system for receiving a light flux from the off-axial focus detection area of the object must also be spaced a sufficient distance from the optical axis. Accordingly, a CCD (charge-coupled device) chip in which a plurality of CCDs, each of which has a plurality of light receiving elements, are formed on a single chip tends to become bulky in size, rendering an AF (auto-focus) module to be correspondingly bulky in size and high in manufacturing cost. On the other hand, if the size of the CCD chip is reduced, the off-axial light flux will become so close to the on-axis light flux as to result in the undesirable interference between these light fluxes, rendering an optical system such as condenser lenses, etc., to be very complicated.
2. Summary of the Invention
Accordingly, the present invention has for its essential object to provide an improved focus detecting device which is very simple in construction and makes it possible for the CCD chip to be manufactured small thereby to make the AF module compact and low in manufacturing cost.
In order to accomplish the above described object, the present invention provides a focus detecting device which comprises an objective lens for forming an image of an object, a condenser lens, located behind a predetermined focal plane of the objective lens, for converging the light passed through the objective lens, a pair of image reforming lenses, located behind the condenser lens, for re-forming a pair of secondary images of the image formed by the objective lens on an off-axial region of the objective lens located at a position far from the optical axis thereof, a light direction changing means, located nearby the condenser lens, for changing the direction along which light bundles forming the secondary images passes towards the optical axis, and a light receiving means for receiving the pair of the secondary images formed by the secondary image forming means to produce a light intensity distribution signal representing the light intensity distribution of the secondary images, a focus condition calculating means for calculating a focus condition of the objective lens by means of detecting the positions of the pair of the secondary images in accordance with the light intensity distribution signal.
According to the present invention, the light flux coming from the off-axial focus detection area which has passed through the objective lens can be directed by the light direction changing means, either before it passes through the condenser lens or after it has passed through the condenser lens, so as to travel towards the optical axis and then to enter the image re-forming lenses. For this purpose, the light direction changing means may be positioned either in front of or rearwardly of the condenser lens with respect to the direction of travel of rays of light towards the image re-forming lens system. The light flux from the off-axial focus detection area, which is incident upon the image re-forming lens system, is focused by the pair of the image re-forming lenses on the light receiving means in the form of a pair of images. Accordingly, it is possible to maintain a substantial distance between the optical axis and the off-axial focus detection area even when the light receiving elements are reduced in size.